1. Field of the Invention
The present invention relates generally to wireless communication systems, and more particularly, to a method and apparatus for transmitting and receiving a random access preamble in a wireless communication system.
2. Description of the Related Art
The terms handover and handoff are generally related to technology that automatically switches a call channel without loss or interruption of service when a mobile communication subscriber moves from one wireless communication area to another.
Random access is a collision-based access procedure that user equipment uses to initially access a base station. In the Third Generation Partnership Project (3GPP) Long Term Evolution (LTE) standard, random access is defined so that user equipment may rapidly access a base station using the least amount of wireless resources. A physical channel used to transmit a random access preamble is a Physical Random Access Channel (PRACH), and the remaining physical channel used for the signaling use is a shared cannel. An LTE system provides a non-contention-based random access procedure to minimize the use of resources and time delays due to collisions.
In performing random access, user equipment transmits a preamble to a base station, using the initial transmission power of the preamble contained in random access information. The random access information, as system information, is downloaded from the base station. When the user equipment successfully transmits the preamble and receives a random access response regarding the preamble from the base station, it can continue performing the procedure of random access. However, if the user equipment does not receive a response from the base station within a certain period of time, it re-transmits the preamble thereto using a transmission power that is higher than that of the previous transmission. This increase is referred to as ‘power ramping.’ The base station downloads the maximum transmission power and the increased amount of transmission power in the power ramping process, as system information, to the user equipment.
FIG. 1 is a diagram illustrating preamble transmission power in a general random access process.
When an event (or a case) for triggering random access has occurred, user equipment transmits a preamble to a base station. As shown in FIG. 1, the events (or cases) include idle to connected mode transition (Case 1), Radio Resource Control (RRC) connection re-establishment (Case 2), handover (Case 3), and DownLink/UpLink (DL/UL) data resume (Case 4). As shown in FIG. 1, the general random access process is performed using the same initial transmission power and ramping power.
If a user equipment and a base station are located near each other, the user equipment can transmit a preamble to the base station using a relatively low level of transmission power. On the contrary, if the user equipment is located at a cell edge furthest from the base station, and transmits a preamble to the base station using a relatively low level of transmission power, the base station may fail to receive the preamble. In that case, the user equipment increases transmission power to transmit the preamble and repeats the re-transmission attempt, which requires more time to perform the random access process.